Linked from
The 9 pages that link to Cretaceous–Paleogene boundary, each with the reason it gives.
CretaceousRelated: This boundary defines the period's end in the rock record.
PaleogeneRelated: It marks the period’s beginning and the end of the Cretaceous.
Late CretaceousRelated: This boundary defines the Late Cretaceous’s upper limit.
Iridium anomalyRelated: The anomaly occurs in sediment layers marking this boundary.
MontanaRelated: Montana’s uppermost Cretaceous deposits preserve fossils close to this boundary.